In an LTE (Long Term Evolution) system, carrier aggregation (CA), where multiple carriers are simultaneously used for communication as a fundamental unit of a predetermined bandwidth (maximum 20 MHz), is adopted. The carrier serving as the fundamental unit in the carrier aggregation is referred to as a component carrier (CC).
When the CA is performed, user equipment is configured with a PCell (Primary Cell) as a cell having high reliability for ensuring connectivity and an SCell (Secondary Cell) as an additional cell. The user equipment can first connect to the PCell and then add the SCell as needed. The PCell is the same cell as a single cell for supporting RLM (Radio Link Monitoring), SPS (Semi-Persistent Scheduling) or the like.
The SCell is a cell configured for the user equipment in addition to the PCell. Addition and deletion of the SCell is conducted by RRC (Radio Resource Control) signaling. The SCell is in a deactivated state immediately after it is configured for the user equipment and accordingly is a cell allowed for communication (scheduling) upon activation.
Also, RLC (Radio Link Control) is used for radio communication between the user equipment and a base station in the LTE system. The RLC can configure any of multiple forwarding modes for each radio bearer. Specifically, there are an RLC-AM (RLC-Acknowledge Mode) where retransmission control is performed based on acknowledgement signals from a receiver side, an RLC-UM (RLC-Unacknowledge Mode) where the retransmission control is not performed, and a TM (Transparent Mode) where the RLC itself is made transparent. The RLC performs communication by exchanging RLC PDU (Protocol Data Units) between an RLC entity in the user equipment side and an RLC entity in the base station side. Also, in the RLC-AM and the RLC-UM, sequence numbers (SNs) attached to a header of the RLC PDU are used for redundancy detection and reordering of the RLC PDUs. The length of the RLC sequence number is specified to have 10 bits in the RLC-AM and 5 or 10 bits in the RLC-UM (see non-patent document 1, for example).